Field of the Invention
This invention relates generally to an exhaust gas diffuser for a gas turbine engine and, more particularly, to an exhaust gas diffuser for a gas turbine engine, where the angular orientation of inlet geometry of the exhaust gas diffuser proximate the last row of blades in the engine is adjustable relative thereto.
Discussion of the Related Art
The world's energy needs continue to rise which provides a demand for reliable, affordable, efficient and environmentally-compatible power generation. A gas turbine engine is one known machine that provides efficient power, and often has application for an electric generator in a power plant, or engines in an aircraft or a ship. A typical gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed airflow to the combustion section where the air is mixed with a fuel, such as natural gas. The combustion section includes a plurality of circumferentially disposed combustors that receive the fuel to be mixed with the air and ignited to generate a working gas. The working gas expands through the turbine section and is directed across turbine blades therein by associated vanes. As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby providing mechanical work.
The turbine section of a typical gas turbine engine will include a plurality of rows of circumferentially disposed blades, such as four rows of blades, where the working gas is directed by a row of vanes across the blades from one stage of the blades to the next stage of the blades. It is generally desirable that the outer tip of the rotating blades be as close as possible to the static casing surrounding the blades, referred to in the art as tip clearance, so that a maximum amount of the working gas as possible flows around the blades instead of flowing between the blades and the casing, which does not contribute to rotation of the blades, to provide improved blade performance. As the temperature of the engine goes up and down, the blades and casings expand and contract accordingly, which changes the tip clearance. Also, the centrifugal force from rotation of the blades causes the length of the blades to increase, which reduces the tip clearance. It is generally the tip clearance of the blades at system steady state operation that determines the performance of the blades and therefore of the engine. On the other hand, the tip clearances are also crucial in ensuring that the blades don't rub with static hardware during the startup and shutdown of the engine because of different thermo-mechanical expansions and/or contractions of blades and casings. Thus, tip clearances are set appropriately in an engine so as to derive the best performance and prevent tip rubbing.
At the output of the turbine section, the working gas is passed through an exhaust diffuser section that modulates the back pressure of the exhausted gas for optimal performance of the turbine section. The exhausted gas, which is still very hot, is often times directed to other systems that may benefit from the available heat until the working gas is eventually exhausted to the environment or otherwise. For example, the hot working gas at the output of the gas turbine engine may be used to boil water for a steam turbine engine, which also generates power in, for example, a combined cycle plant, well known to those skilled in the art. The configuration of the exhaust gas diffuser at the output of the gas turbine engine is important for the performance of the gas turbine blades because the exhaust gas diffuser partially blocks the gas flow from the turbine section.
The performance of an exhaust gas diffuser is measured by its coefficient of pressure recovery. More particularly, an exhaust gas diffuser converts kinetic energy in the exhaust gas into potential energy, where the exhaust gas diffuser acts to reduce the speed of the working gas, preferably to zero speed so that all of the kinetic energy in the working gas is converted to a pressure. The angular orientation of the exhaust gas diffuser at its inlet geometry proximate the last row of blades is important for adequately collecting the working gas, which contributes to the performance of the exhaust gas diffuser. It is known that as the tip clearance of the last row of blades in the engine is reduced to be less than some threshold clearance, where reducing the tip clearance continues to increase the performance of the blades, the performance of the exhaust gas diffuser is reduced. Thus, for very small tip clearances, the ability of the exhaust gas diffuser to provide pressure recovery of the exhaust gas is reduced. Therefore, the combination of the last stage blade tip clearance and the angle of the inlet geometry of the exhaust gas diffuser is a critical contributor to optimal engine performance and efficiency.
The inlet geometry of most exhaust gas diffusers have a fixed angle that is set for optimal performance for a general ambient temperature operating condition of the engine. However, that angular orientation of the exhaust gas diffuser may only be optimal for an average ambient temperature and not be optimal for the typical ambient temperature of the service location of the engine. More particularly, when a gas turbine engine operates at an off-design condition, such as cold or hot day or part load, two effects can be discerned, namely, the running blade tip clearance deviates from its optimum clearance and flow conditions change. These two effects degrade system performance.